When the optical elements (such as processing lens, PR mirror or window) through which the laser beam passes through the laser cutting machine are stained, the thermal lens effect will occur after the laser beam passes through, and the cutting surface will become rough as a result, slag will appear, and even overburn will occur. When the processing quality of the laser cutting machine becomes worse, to judge whether it is affected by the thermal lens effect, first check whether the performance of the optical elements changes with the irradiation time of the laser beam. At the beginning of processing, the optical elements are still in a low temperature state. With the progress of processing, the influence of thermal lens effect will be shown from the cutting surface quality
reason
Reflective optical elements (such as refractors) can be cooled efficiently directly from the back, and the reflective optical elements will hardly have thermal mirror effect; The transparent optical element can only be cooled indirectly by cooling its surroundings, and both sides of the optical element will generate heat, so it is easy to generate thermal lens effect. When the temperature of optical elements rises, thermal deformation will occur, and the refractive index of the laser beam of the laser cutting machine will also change, so will the focusing characteristics of the laser beam.
Resolution method
Compare the cutting surface at the beginning and end of the workpiece processed by the laser cutting machine. At the beginning of the processing, due to the short irradiation time of the laser beam, the optical elements are still in the low temperature state, so the influence of the thermal lens effect is relatively small. At the end of processing where the cutting time needs more than 10s, thermal lens effect will occur because the optical element is irradiated by the laser beam for a long time, and the focus position and beam mode will also change.
Once the focus position changes or the beam mode changes, it will not automatically return to the original state during continuous irradiation processing. The cutting surface at the beginning and end of the machining is compared, and the quality at the end of the machining becomes worse Then thermal lens effect occurs; If the two parts are in good condition with the same quality, it means that there is no thermal lens effect.
When laser cutting machine is used to process stainless steel materials and judge whether thermal lens effect has occurred, it is necessary to observe the slag hanging condition at the beginning and end of the processing. If the burr amount at the processing place increases, it indicates that thermal lens effect may have occurred. In addition, in order to exclude the directivity caused by other reasons such as the non-uniformity of the beam mode or the inclination of the optical axis, it should be noted that the same cutting surface (the same processing direction) must be suspended for comparison.